This application is based on Japanese Patent Application Nos. 11-189334 and 11-220557 respectively filed on Jul. 2, 1999 and Aug. 3, 1999 in Japan, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display in which a plurality of liquid crystal light control layers is stacked. In particular, the present invention relates to a reflective type liquid crystal color display having a plurality of liquid crystal light control layers, each of which includes a liquid crystal light control material exhibiting a cholesteric phase.
2. Description of the Related Art
In recent years, research and development of thin, low-power-consumption and bright reflective type color displays has been performed. Of such displays, a reflective type color display using a selective reflection characteristic of a cholesteric liquid crystal material receives attention. In the reflective type color display, the following layers are stacked: a liquid crystal light control layer having the peak of the selective reflection wavelength in the wavelength region of red liquid crystal light control layer); a liquid crystal light control layer having the peak of the selective reflection wavelength in the wavelength region of green (G liquid crystal light control layer); and a liquid crystal light control layer having the peak of the selective reflection wavelength in the wavelength region of blue (B liquid crystal light control layer). The reflective type color display achieves high brightness because it does not use a polarizing plate. In addition, since the cholesteric liquid crystal material exhibits a memory effect, the reflective type color display is low in power consumption, can be simple-matrix-driven, and can be manufactured inexpensively. For these reasons, the reflective type color display receives attention.
However, since the selective reflection wavelength of cholesteric liquid crystal has angle dependence in principle, the color tone varies according to the viewing position (viewing angle). With respect to this problem, Japanese Laid-open Patent Publication No. 10-31210A proposes a reflective type color display in which a B liquid crystal light control layer, a G liquid crystal light control layer and an R liquid crystal light control layer are stacked in this order from the extraneous light incident side. In this reflective type color display, a filter layer absorbing blue light and transmitting green light and red light is provided between the B liquid crystal light control layer and the G liquid crystal light control layer, and a filter layer absorbing blue light and green light and transmitting red light is provided between the G liquid crystal light control layer and the R liquid crystal light control layer.
However, the transmittance of the filter layers shown in that publication is substantially zero in the wavelength regions absorbed by the filter layers (see FIG. 4 of the publication). In a display provided with such filter layers, the transmittance is as low as approximately 80% even in the wavelength regions to be transmitted by the filters, so that the display screen is extremely dark.
Moreover, the wavelength regions absorbed by the filter layers are extremely close to the selective reflection wavelengths of the liquid crystal light control layers. Therefore, only by changing the viewing angle several degrees from the front, the selective reflection wavelengths of the liquid crystal light control layers shift into the regions absorbed by the filter layers, so that the display on the screen disappears. As described above, a problem of reduction in viewing angle characteristic arises in the color display of the prior art taught by the publication.
Apart from the above-mentioned problem, a multilayer liquid crystal display in which a plurality of liquid crystal light control layers using a liquid crystal material exhibiting a cholesteric phase is stacked is subject to various limitations in order to provide display with excellent color balance. In particular, the reflection characteristic of each of the stacked liquid crystal light control layers and the transmission characteristics of members such as the substrates sandwiching the liquid crystal light control layers and the electrodes formed on the substrates are extremely important. As an example, an RGB multilayer liquid crystal display in which an R liquid crystal light control layer, a G liquid crystal light control layer and a B liquid crystal light control layer are stacked in this order from the viewing side will be examined.
In the multilayer liquid crystal display, light reflected from the B liquid crystal light control layer disposed farthest from the viewing side cannot be viewed unless it passes through the two R and G liquid crystal light control layers disposed on the viewing side of the B liquid crystal light control layer and the members such as the substrates sandwiching the liquid crystal light control layers and the electrodes formed on the substrates. Therefore, the use efficiency of the light incident on the B liquid crystal light control layer and the light reflected at the B liquid crystal light control layer is low. Consequently, when the liquid crystal light control layers have the same reflectance, red is comparatively intense and blue is comparatively weak in the display, so that the color balance is inferior. When indium tin oxide (ITO) electrodes, which have the property of absorbing blue light, are used as the electrodes, the display characteristic of blue further deteriorates.
Moreover, as mentioned above, the selective reflection of cholesteric phase liquid crystal has viewing angle dependence. Therefore, when light is obliquely incident on the display screen or when the display screen is viewed from a slanting direction, the selective reflection wavelength shifts toward the shorter wavelength side, so that the hue of the displayed color changes. The amount of the shift increases as the selective reflection wavelength increases. In the above-described RGB multilayer liquid crystal display, in a case where the half widths of the liquid crystal light control layers are substantially the same, when the display screen is viewed from a slanting direction, the selective reflection wavelength of the R liquid crystal light control layer shifts comparatively substantially, so that the hue of the displayed red significantly changes. As a result, it is difficult to provide excellent display.
Accordingly, a primary object of the present invention is to provide an improved multilayer liquid crystal color display.
Another object of the present invention is to provide a liquid crystal display in which reduction in contrast is suppressed so that the display screen does not become dark.
Yet another object of the present invention is to provide a liquid crystal display in which reduction in contrast is suppressed so that the viewing angle characteristic does not deteriorate
Still another object of the present invention is to provide a liquid crystal display in which deterioration in viewing angle characteristic and reduction in the brightness of the display screen are suppressed.
A still further object of the present invention is to provide a multilayer liquid crystal display in which a plurality of liquid crystal light control layers is stacked, wherein the light use efficiency is high, the change in hue caused when the viewing direction is changed is small and the color balance of the displayed color is excellent.
To achieve at least one of the above-mentioned objects, a liquid crystal display according to a first aspect of the present invention comprises: a plurality of liquid crystal light control layers each of which includes liquid crystal exhibiting a cholesteric phase and having a peak wavelength of a selective reflection wavelength range of a reflection spectrum in the visible wavelength range, the liquid crystal light control layers being stacked in such order that the peak wavelength of any of the liquid crystal light control layers is larger than that of the adjoining liquid crystal light control layer on the viewing side, wherein a half width of the reflection spectrum of any of the liquid crystal light control layers is larger than that of the adjoining liquid crystal light control layer on the viewing side, wherein a maximum reflectance of the reflection spectrum of any of the liquid crystal light control layers is higher than that of the adjoining liquid crystal light control layer on the viewing side, and wherein the chromaticity coordinate position, in the XYZ calorimetric system, of a color displayed when all the liquid crystal light control layers are in a state of reflecting at the maximum reflectance is present in a range within a distance of 0.02 from the chromaticity coordinate position of the standard white point.
In the liquid crystal display having the above-described structure, each of the liquid crystal light control layers may be sandwiched between a pair of substrates. The electrodes may also be formed on each of the substrates. The substrate arrangement may be as follows: Only one substrate (common substrate) is disposed between two adjoining liquid crystal light control layers and the common substrate is used for holding the two liquid crystal light control layers. The electrodes may be formed directly on the substrate or may be formed on the substrate with another layer (for example, a subsequently-described filter layer) in between. Another layer (for example, an insulating layer or the subsequently-described filter layer) may be provided on the electrodes.
In the liquid crystal display having the above-described structure, the substrates are, for example, all transparent substrates. The substrate situated farthest from the viewing side is not necessarily transparent. Examples of the transparent substrate material include resins such as polyethylene terephthalate, polycarbonate and polyether sulfone, and glass. It is desirable that the transparent substrates have sufficient transmittance in the visible wavelength region. The electrodes which the multilayer liquid crystal display is provided with are, for example, all transparent electrodes. The electrode disposed farthest from the viewing side is not necessarily transparent. Examples of the transparent electrodes include ones made of a material such as ITO, SnO2 or InO3, and ones made of thin metal films. The electrodes can be formed on the substrates, for example, by a sputtering method, a vacuum evaporation method or a printing method.
Examples of the liquid crystal, exhibiting a cholesteric phase, of the liquid crystal light control layers include cholesteric liquid crystal, and chiral nematic liquid crystal formed by adding a chiral agent to nematic liquid crystal so that a desired helical pitch is obtained. The liquid crystal light control layers may include, for example, spacers for adjusting the thickness of the liquid crystal (the substrate-to-substrate gap) in addition to the liquid crystal exhibiting a cholesteric phase.
The liquid crystal light control layers can be formed, for example, by filling, by a vacuum filling method, liquid crystal exhibiting a cholesteric phase into the space surrounded by the substrates and a sealing wall provided on a peripheral part between the substrates. Moreover, the liquid crystal light control layers can be formed, for example, by forming the sealing wall on one of the substrates, dropping liquid crystal exhibiting a cholesteric phase onto either one of the substrates and superposing the other substrate on the substrate.
In the multilayer liquid crystal display of the present invention described above, for example, three liquid crystal light control layers, i.e., an R liquid crystal light control layer, a G liquid crystal light control layer, and a B liquid crystal light control layer, are stacked. In the description that follows, the multilayer liquid crystal display will be referred to as an RGB multilayer liquid crystal display.
A filter means may be provided on the viewing side of or in at least one of the liquid crystal light control layers for the purpose of absorbing at least part of light components in a wavelength range shorter than the selective reflection wavelength range of the at least one of the liquid crystal light control layers. By providing the filter means, the color purity of the at least one liquid crystal light control layer is improved.
The filter means may be selected from a filter layer provided on the viewing side of the at least one of the liquid crystal light control layer, and a coloring agent added to the at least one of the liquid crystal light control layer.
The filter means does not necessarily absorb light of all the wavelengths shorter than the selective reflection wavelength range of the at least one of the liquid crystal light control layers. For example, it is necessary for the filter means only to absorb light of wavelengths in the visible region shorter than the selective reflection wavelength range of the liquid crystal light control layer. The filter layer may be disposed, for example, between the electrodes on the viewing side of the at least one of the liquid crystal light control layers and the at least one of the liquid crystal light control layers or between the electrode on the viewing side of the at least one of the liquid crystal light control layers and the substrate where the electrodes are formed. The filter layer can be formed, for example, by applying a light absorbing material (filter material) to a predetermined surface by a spin coating method or a printing method. The filter layer can also be formed by pasting a film made of a light absorbing material (filter material) onto a predetermined surface. The reason why such a filter means is provided will be mentioned later. It is desirable to provide the filter means for each of the liquid crystal light control layers.
In the above-described multilayer liquid crystal display of the present invention, a plurality of liquid crystal light control layers is stacked in such order that the peak wavelength of the selective reflection wavelength range of the reflection spectrum of any of the liquid crystal light control layers is larger than that of the adjoining liquid crystal light control layer on the viewing side. That is, when the multilayer liquid crystal display has a number n (n is an integer not less than 2) of stacked liquid crystal light control layers, the following expression 1 is satisfied:
xcex1 less than xcex2 less than . . .  less than xcexnxe2x80x83xe2x80x83(1)
where the peak wavelengths of the liquid crystal light control layers are xcex1, xcex2, . . . , xcexn from the viewing side.
That is, the farther from the viewing side a liquid crystal light control layer is, the larger the peak wavelength of the liquid crystal light control layer is. In the above-described RGB multilayer liquid crystal display, the liquid crystal light control layers are disposed in the order of the B liquid crystal light control layer, the G liquid crystal light control layer and the R liquid crystal light control layer from the viewing side. The reason why the relationship of the expression 1 is satisfied will be mentioned later.
Moreover, the half width of the reflection spectrum of any of the liquid crystal light control layers is larger than that of the adjoining liquid crystal light control layer on the viewing side. That is, when the multilayer liquid crystal display has a number n (n is an integer not less than 2) of stacked liquid crystal light control layers, the following expression 2 is satisfied:
xcex94xcex1 less than xcex94xcex2 less than . . .  less than xcex94xcexnxe2x80x83xe2x80x83(2)
where the half widths of the reflection spectrums of the liquid crystal light control layers are xcex94xcex1, xcex94xcex2, . . . , xcex94xcexn from the viewing side.
That is, the farther from the viewing side a liquid crystal light control layer is, the larger the half width of the reflection spectrum of the liquid crystal light control layer is. The reason why the relationship of the expression (2) is satisfied will be mentioned later. The half width xcex94xcex of the reflection spectrum of the liquid crystal light control layer is (xcexHxe2x88x92xcexL). The wavelengths xcexH and xcexL (xcexH greater than xcexL) are wavelengths where the reflectances of the liquid crystal light control layer are xc2xd the reflectance when the liquid crystal light control layer reflects light of the peak wavelength of the reflection spectrum. In the description that follows, the half width of the reflection spectrum will sometimes be referred to merely as half width.
Moreover, the maximum reflectance of any of the liquid crystal light control layers is higher than that of the adjoining liquid crystal light control layer on the viewing side. That is, when the multilayer liquid crystal display has a number n (n is an integer not less than 2) of stacked liquid crystal light control layers, the following expression 3 is satisfied:
%R1 less than %R2 less than . . .  less than %Rnxe2x80x83xe2x80x83(3)
where the maximum reflectances of the liquid crystal light control layers are %R1, %R2, . . . , %Rn from the viewing side.
That is, the farther from the viewing side a liquid crystal light control layer is, the higher the maximum reflectance of the liquid crystal light control layer is. The reason why the relationship of the expression (3) is satisfied will be mentioned later.
The peak wavelengths to xcex1 to xcexn, the half widths xcex94xcex1 to xcex94xcexn and the maximum reflectances %R1 to %Rn are values measured in a condition where the liquid crystal light control layers are not stacked.
In the above-described multilayer liquid crystal display of the present invention, the chromaticity coordinate position P of a color displayed when all the liquid crystal light control layers are in a state of reflecting at the maximum reflectance is present in a range within a distance of 0.02 from the chromaticity coordinate position of the standard white point. That is, the distance d between the chromaticity coordinate position P of the color displayed when all the liquid crystal light control layers are in a state of reflecting at the maximum reflectance and the chromaticity coordinate position P0 of the standard white point is not more than 0.02. The chromaticity coordinate position P(x, y) of the displayed color, the chromaticity coordinate position P0(x0, y0) of the standard white point and the distance d are all in the XYZ colorimetric system. That is, the following expression 4 is satisfied:
d=((xxe2x88x92x0)2+(yxe2x88x92y0)2)xe2x89xa60.02 . . . (4)
The chromaticity coordinate position of the displayed color is measured by using as the light source a standard illuminant D65, a standard illuminant A, a standard illuminant C, a fluorescent lamp F2, a fluorescent lamp F6, a fluorescent lamp F7, a fluorescent lamp F8, a fluorescent lamp F10 or a fluorescent lamp F11. It is necessary only that the expression 4 be satisfied when measurement is performed by use of any one of these light sources. The chromaticity coordinate position of the standard white point corresponds to the light source used for the measurement. For example, when the standard illuminant D65 is used, the chromatic coordinate position P0 of the standard white point is (x0, y0)=(0.3127, 0.3290), and when the standard illuminant A is used, the chromatic coordinate position P0 of the standard white point is (x0, y0)=(0.4476, 0.4074).
That the expression 4 is satisfied indicates that the color displayed when all the liquid crystal light control layers are in a state of reflecting at the maximum reflectance is white and that the hue of the displayed white is excellent. Since the expression 4 is satisfied when all the liquid crystal light control layers are in a state of reflecting at the maximum reflectance, the satisfaction of the expression 4 is advantageous when the multilayer liquid crystal display is controlled by a display drive circuit.
In the above-described RGB multilayer liquid crystal display, that the color displayed when all the liquid crystal light control layers are in a state of reflecting at the maximum reflectance is white and that the displayed white satisfies the expression 4 indicate that white is displayed with red, green and blue being well-balancedly mixed.
While a multiplicity of combinations of liquid crystal light control layers satisfying the expression 4 is possible in multilayer liquid crystal displays, in the multilayer liquid crystal display of the present invention, since the expressions 1 to 3 are also satisfied and the filter layer is provided, color purity is excellent also for the displayed color of each individual liquid crystal light control layer, mixed colors (including white) and neutral tints.
The reason why the expressions 1 to 3 are satisfied will be described. The reason why the filter layer is provided will also be described.
In the multilayer liquid crystal display of the present invention, the liquid crystal light control layers each include liquid crystal exhibiting a cholesteric phase as mentioned above. The reflection spectrum of a liquid crystal light control layer when the liquid crystal of the liquid crystal light control layer is in planar state roughly depends on the cholesteric pitch (helical pitch), the refractive index anisotropy and the thickness (substrate-to-substrate gap) of the liquid crystal. The cholesteric pitch, the refractive index anisotropy and the liquid crystal thickness affect the reflection spectrum, the half width and the maximum reflectance of the liquid crystal light control layer under perfect diffuse illumination, respectively. In order for a multilayer liquid crystal display to provide display with excellent color purity, etc., it is necessary that the characteristic values of each of the liquid crystal light control layers be well matched.
When chiral nematic liquid crystal formed by adding a chiral agent to nematic liquid crystal is used as the liquid crystal, exhibiting a cholesteric phase, of the liquid crystal light control layers, the cholesteric pitch of the liquid crystal can be adjusted by the amount of addition of the chiral agent that supplies a twisting force to nematic liquid crystal which is the host liquid crystal. By increasing the adding amount of the chiral agent, the twisting force increases to shorten the cholesteric pitch, so that the selective reflection wavelength is shortened. The selective reflection wavelength xcex can be represented by the following expression 5:
xcex=nxc3x97Pxe2x80x83xe2x80x83(5)
Here, n is the average refractive index of the liquid crystal, and P is the cholesteric pitch that can be adjusted by the adding amount of the chiral agent.
The refractive index anisotropy of chiral nematic liquid crystal that affects the half width of the liquid crystal light control layer is largely dependent on the refractive index anisotropy of nematic liquid crystal which is the host liquid crystal, and is reduced by adding the chiral agent. The half width xcex94xcex can be represented by the following expression (6):
xcex94xcex=xcex94nxc3x97Pxe2x80x83xe2x80x83(6)
Here, xcex94n is the refractive index anisotropy of chiral nematic liquid crystal, and P is the cholesteric pitch that can be adjusted by the amount of addition of the chiral agent.
The liquid crystal thickness (liquid crystal light control layer thickness) corresponds to the gap between the substrates sandwiching the liquid crystal (electrode-to-electrode gap). When spherical spacers are disposed between the substrates, the liquid crystal thickness can be adjusted by the diameter of the spacers. When the liquid crystal thickness is small, the number of helical structures that contribute to reflection is small, so that the maximum reflectance when the liquid crystal is in planar state is low. On the contrary, when the liquid crystal thickness is increased, the maximum reflectance when the liquid crystal is in planar state can be increased. Here, the number of helical structures is the number of cholesteric pitches aligned in the direction of the normal to the electrode surface (direction of the normal to the substrate surface).
A feature when liquid crystal exhibiting a cholesteric phase performs selective reflection is the transmittivity of light of wavelengths other than the wavelengths in the selective reflection wavelength region. When the transmittance of light of a wavelength longer than the selective reflection wavelength region of a liquid crystal light control layer and the transmittance of light of a wavelength shorter than the selective reflection wavelength region of the liquid crystal light control layer are compared, the transmittance of light of the longer wavelength is higher. Therefore, by disposing the liquid crystal light control layers so that liquid crystal light control layers having smaller selective reflection wavelengths are closer to the viewing side, the light use efficiency can be increased. That is, to increase the light use efficiency, the liquid crystal light control layers are stacked so as to satisfy the expression 1. In other words, the light use efficiency can be increased by stacking the liquid crystal light control layers so as to satisfy the expression 1. When ITO transparent electrodes are used as the electrodes, since ITO electrodes have the property of absorbing light in the wavelength region of blue (shorter wavelength region in the visible region), even though this property is considered, the liquid crystal light control layers are stacked so as to satisfy the expression 1.
For example, in the above-described RGB multilayer liquid crystal display, the liquid crystal light control layers are stacked in the order of the B liquid crystal light control layer, the G liquid crystal light control layer and the R liquid crystal light control layer from the viewing side. In the RGB multilayer liquid crystal display, even though ITO electrodes are used, since the B liquid crystal light control layer is disposed in the position closest to the viewing side, visible light can reach the B liquid crystal light control layer after passing through an ITO electrode only once. Therefore, compared to a case where the B liquid crystal light control layer is disposed in another position, blue is excellently displayed. Consequently, the user can visually recognize blue with ease, and colors displayed by mixing blue with other colors can also be displayed excellently.
There is another reason why the liquid crystal light control layers are stacked so as to satisfy the expression 1. This will be mentioned later.
In the multilayer liquid crystal display, it is desirable that a liquid crystal light control layer with a longer selective reflection wavelength (a longer peak wavelength) have a larger half width. This is for the following reason: The selective reflection of liquid crystal exhibiting a cholesteric phase has viewing angle dependence, and when light is obliquely incident on the display screen or the display screen is viewed from a slanting direction, the selective reflection wavelength shifts toward the shorter wavelength side, so that the hue of the displayed color changes. The amount of the shift increases as the selective reflection wavelength increases. However, when the shift amount is the same, the change in hue decreases as the half width increases. Although the change in hue decreases as the half width of the liquid crystal light control layer increases, since the purity of the color displayed by the liquid crystal light control layer alone deteriorates, the increase in half width is limited. Considering both the change in hue and the purity of the displayed color, the liquid crystal light control layers are set so that liquid crystal light control layers having longer selective reflection wavelengths have larger half widths as mentioned above. In the multilayer liquid crystal display of the present invention, since a plurality of liquid crystal light control layers is stacked so as to satisfy the expression 1 associated with the selective reflection wavelength, the liquid crystal surface layers are set so as to satisfy the expression 2 associated with the half width. By satisfying the expression 2, the change in the hue of the displayed color of each liquid crystal light control layer can be suppressed and the deterioration in the purity of the displayed color of each liquid crystal light control layer can be suppressed.
With respect to the maximum reflectance of a liquid crystal light control layer, considering the loss caused when light passes through another liquid crystal light control layer disposed on the viewing side of the liquid crystal light control layer and considering the balance among component colors when mixed colors (including white) are displayed, it is preferable that the farther from the viewing side a liquid crystal light control layer is, the higher the maximum reflectance of the liquid crystal light control layer is. That is, it is desirable that the expression 3 be satisfied. In the multilayer liquid crystal display of the present invention described above, since the expression 3 is satisfied, the balance among component colors when mixed colors are displayed is excellent accordingly, so that the mixed colors are excellent in hue.
The filter means (for instance, the filter layer) may be optionally provided for the following reason: When liquid crystal exhibiting a cholesteric phase performs selective reflection as described above, the transmittance of light of a wavelength shorter than the selective reflection wavelength region is lower than that of light of a longer wavelength. This is because of scattering within the liquid crystal exhibiting a cholesteric phase. When the reflection spectrum is observed, light of wave lengths shorter than the selective reflection wavelength region is viewed as scattered light. For example, in the above-described RGB multilayer liquid crystal display, scattering in the R liquid crystal light control layer is conspicuously observed. The R liquid crystal light control layer scatters light of the wavelength regions of green and blue as well as selectively reflects light of the wavelength region of red. Because of the scattering, the color purity of red is apt to deteriorate, and the viewability of red deteriorates. By cutting the scattered light through absorption, the purity of red can be improved. Therefore, as mentioned above, in the R liquid crystal light control layer, a red filter layer is provided that absorbs light of the wavelength regions of green and blue shorter than the selective reflection wavelength region (wavelength region of red). Likewise, in the G liquid crystal light control layer, a yellow filter layer is provided that absorbs light of the wavelength region of blue shorter than the selective reflection wavelength region (wavelength region of green). Moreover, in the B liquid crystal light control layer, an ultraviolet cut filter layer (ultraviolet absorbing filter layer) is provided that absorbs ultraviolet rays of the wavelength region shorter than the selective reflection wavelength region (wavelength region of blue). In the RGB multilayer liquid crystal display, since the B liquid crystal light control layer is disposed in the position closest to the viewing side as mentioned above, the deterioration of the liquid crystal of each liquid crystal light control layer, etc., can also be suppressed by the ultraviolet cutting filter layer provided for the B liquid crystal light control layer. It is necessary to provide the filter layer for only at least one liquid crystal light control layer as mentioned above. For example, in the RGB multilayer liquid crystal display, the filter layer may be provided for only the R liquid crystal light control layer significantly affected by the scattered light. Each of the filter layers also have the effect of decreasing the change in hue of the displayed color (angle dependence) caused, for example, when the display screen is viewed from a slanting direction.
In the multilayer liquid crystal display of the present invention, since the liquid crystal light control layers are stacked so as to satisfy the expression 1, even though the filter layer for absorbing the light of wavelength regions shorter than the selective reflection wavelength region of the liquid crystal light control layer is provided for any one or more than one of a plurality of liquid crystal light control layers, the filter layer does not absorb the light components necessary for the liquid crystal light control layer situated farther from the viewing side than the liquid crystal light control layer for which the filter layer is provided. In other words, in a case where the liquid crystal light control layers are stacked so as not to satisfy the expression 1, when a filter layer as described above is provided for a liquid crystal light control layer, there are occasions when the filter layer absorbs light of the wavelength region necessary for the liquid crystal light control layer situated farther from the viewing side than the liquid crystal light control layer for which the filter layer is provided.
Therefore, to increase the light use efficiency as mentioned above and to provide the filter layer for increasing the color purity of the displayed color of each liquid crystal light control layer, it is necessary to satisfy the expression 1. Moreover, since provision of such a filter layer for each liquid crystal light control layer makes it unnecessary to consider scattered light when the display characteristic of the multilayer liquid crystal display is optimized, designing is facilitated.
A liquid crystal display according to a second aspect of the present invention comprises: a plurality of liquid crystal light control layers each of which includes liquid crystal exhibiting a cholesteric phase and having a peak wavelength of a selective reflection wavelength range of a reflection spectrum in the visible wavelength range, the liquid crystal light control layers being stacked in such order that the peak wavelength of any of the liquid crystal light control layers is larger than that of the adjoining liquid crystal light control layer on the viewing side; and filter means provided on the viewing side of or in at least one of the liquid crystal light control layers, wherein a transmittance of the filter means is 10 to 70% in the selective reflection wavelength range of the liquid crystal light control layer situated on the viewing side of than the filter layer.
By the transmittance of the filter means being 10 to 70% in the selective reflection wavelength range of the liquid crystal light control layer situated on the viewing side than the filter means as described above, a liquid crystal display can be obtained in which the display screen does not become dark with contrast deterioration being suppressed.
A liquid crystal display according to a third aspect of the present invention comprises: a plurality of liquid crystal light control layers each of which includes liquid crystal exhibiting a cholesteric phase and having a peak wavelength of a selective reflection wavelength range of a reflection spectrum in the visible wavelength range, the liquid crystal light control layers being stacked in such order that the peak wavelength of any of the liquid crystal light control layers is larger than that of the adjoining liquid crystal light control layer on the viewing side; and filter means provided on the viewing side of or in at least one of the liquid crystal light control layers, wherein the filter means absorbs light components in a shorter wavelength range from the peak wavelength of the liquid crystal light control layer situated not closer to the viewing side than the filter means to 1.3 to 1.5 times of the half width of the reflection spectrum of the liquid crystal light control layer situated not closer to the viewing side than the filter layer, wherein a transmittance of the filter means in the shorter wavelength range is 10 to 70%.
By thus defining the transmittance of the filter means, a liquid crystal display can be obtained in which deterioration in contrast and reduction in viewing angle are suppressed.
A liquid crystal display according to a fourth aspect of the present invention comprises: a plurality of liquid crystal light control layers each of which includes liquid crystal exhibiting a cholesteric phase and having a peak wavelength of a selective reflection wavelength range of a reflection spectrum in the visible wavelength range, the liquid crystal light control layers being stacked in such order that the peak wavelength of any of the liquid crystal light control layers is larger than that of the adjoining liquid crystal light control layer on the viewing side; filter means provided on the viewing side of or in at least one of the liquid crystal light control layers, wherein the filter means absorbs light components in a shorter wavelength range of 120 to 150 nm from the peak wavelength of the liquid crystal light control layer situated not closer to the viewing side than the filter means, and wherein a transmittance in the shorter wavelength range is 10 to 70%.
By thus defining the transmittance of the filter means, a liquid crystal display can be obtained in which reduction in viewing angle and deterioration in the brightness of the display screen are suppressed.
In the liquid crystal displays according to the second to fourth aspects, similar to the first aspect of the present invention, the filter means may be in a form of (1) a filter layer provided on the viewing side of the at least one of the liquid crystal light control layers, or (2) a coloring agent added in the at least one of the liquid crystal light control layers.
In the liquid crystal displays according to the second to fourth aspects, the transmittance of the filter means in the shorter wavelength range is 10 to 70% may be a flat region of the spectral transmittance characteristic of the filter layer, a falling region in which the transmittance steeply decreases, or a region including both of these.
In the liquid crystal displays according to the second to fourth aspects, each of the liquid crystal light control layers may be sandwiched between substrates. With a total of two substrates, one holding one liquid crystal light control layer and the other holding the other liquid crystal light control layer, being present between adjoining liquid crystal light control layers, the filter layer may be provided between these two substrates. The filter layer may be provided on the side of the substrate opposite to the side which is in contact with the liquid crystal material. By doing this, the liquid crystal material and the filter layer are physically separated. Consequently, ingredients of the filter layer are prevented from melting into the liquid crystal material, so that a liquid crystal display in which the display characteristic is not degraded is obtained.
Further, in the liquid crystal displays according to the second to fourth aspects, the filter layer may be provided on the side of the substrate which is in contact with the liquid crystal material. In this case, the filter layer effectively suppresses scattering of light by the substrate.
Further, in the liquid crystal displays according to the second to fourth aspects, the filter layer may be provided between the substrate and an electrode provided on the surface, opposed to the liquid crystal light control layer, of each of the substrates sandwiching the liquid crystal light control layer. By doing this, the voltage applied to the liquid crystal light control layer can be prevented from decreasing due to the filter layer, so that the liquid crystal display can be driven at a low voltage. Moreover, scattering of light by the substrate can be effectively suppressed.
The filter layer may be provided on the electrode provided on each substrate. By doing this, scattering of light by the substrate can be effectively suppressed. Moreover, the filter layer can be used as a functional film such as an insulating film, which is effective in simplifying the device structure.
The filter layer may be provided on the side of the substrate opposite to the electrode formed side. This is advantageous in driving the liquid crystal display at a low voltage.
Further, in the liquid crystal display according to the present invention, by the substrates being made of resin films, reduction in brightness due to the stacking of a plurality of liquid crystal light control layers can be suppressed.
Further, in the liquid crystal display according to the present invention, by providing the sealing wall on a peripheral part of the pair of substrates sandwiching each liquid crystal light control layer, not only the liquid crystal material can be easily filled into the gap between the substrates but also the liquid crystal is prevented from being in contact with outside air, so that deterioration in display characteristic can be reduced.
Further, in the liquid crystal display according to the present invention, by providing a resin structure bonding and supporting the substrates sandwiching each liquid crystal light control layer at least in a predetermined position in the display region, the substrate-to-substrate gap which decides the thickness of each liquid crystal light control layer can be maintained uniform, so that a liquid crystal display in which display unevenness is extremely small can be fabricated.
Further, in the liquid crystal display according to the present invention, by disposing spacers between the pair of substrates sandwiching each liquid crystal light control layer, the substrate-to-substrate gap can be defined by the diameter of the spacers, so that a liquid crystal display in which substrate-to-substrate gap unevenness is small can be fabricated.
Further, when the spacers are fixed-by-adhesion spacers, since the spacers do not readily move even when large-size substrates or film substrates are used, variation in substrate-to-substrate gap is small and display unevenness does not readily vary with time.